Moving cast shadows induce apparent motion in depth

We describe phenomenally strong visual illusions in which the motion of an object's cast shadow determines the perceived 3D trajectory of the object. Simply adjusting the motion of a shadow is sufficient to induce dramatically different apparent trajectories of the object casting the shadow. Using 3D graphics, we report psychophysical results which show that: 1) the information provided by the motion of an object's shadow overrides other strong sources of information and perceptual biases, such as the assumption of constant object size and a general viewpoint; 2) the natural constraint of shadow darkness plays a role in the interpretation of a moving image patch as a shadow, but under some conditions even unnatural light shadows can induce apparent motion in depth of an object; 3) when shadow motion is caused by a moving light source, the visual system incorrectly interprets the shadow motion as consistent with a moving object, rather than a moving light source. The results support the hypothesis that the human visual system incorporates a stationary light source constraint in the perceptual processing of spatial layout of scenes